1. Field of the Invention
The present invention relates to a fuel supply amount control apparatus for an internal combustion engine for controlling a fuel amount to be injected into the internal combustion engine, and in particular to one which determines a fuel injection amount in consideration of the behavior of fuel injected into an intake system.
2. Description of the Related Art
In the prior art, the apparatus disclosed in Unexamined Japanese Patent Application Publication No. H1-216042 and the apparatus etc. disclosed in Unexamined Japanese Patent Application Publication No. H4-252833, for example, are known as such type of control apparatus, i.e. a control apparatus for controlling a fuel supply amount to an internal combustion engine based on the behavior of the fuel injected into the intake system thereof.
Either of these fuel supply amount control apparatuses computes a residual fuel amount remaining in the intake system at the next fuel injection time using a fuel behavior model for expressing as an equation the behavior of fuel injected into the intake system of the internal combustion engine from a fuel injection valve when it is introduced into a cylinder while evaporating due to the opening of an intake valve. The injected fuel amount to actually be injected is computable if the residual fuel amount from the previous injection remaining in the intake system at the time of the next injection of fuel is determined, since the fuel amount required for the internal combustion engine is measurable based on the operating conditions of the internal combustion engine and the target value of the air-fuel ratio thereof is measurable.
As a fuel behavior model described above, there is the following known equation (1). This equation is established based on the two points that a fuel amount MF(t) remaining in the intake system at a given time corresponds to the addition of an injected fuel amount GF during one stroke to a remaining fuel amount not introduced into the cylinder in the previous injected fuel amount, and that a large portion of the fuel injected into the intake system adheres to the inner wall of the intake system and is introduced into the cylinder in order while evaporating together with the opening of the intake valve, thus this remaining fuel amount can be understood as the amount of chronological or time-dependent change based on a given time constant. EQU MF(t)=MF(t-.DELTA.t).circle-solid.e.sup.-.DELTA.t/.tau. +GF (1)
In this equation (1) .tau. is a time constant (herebelow referred to as "evaporation time constant") indicating chronological change in the fuel amount introduced into the cylinder from the intake system of the internal combustion engine after injection of fuel by the fuel injection valve, .DELTA.t is time corresponding to a general crank angle of 720.degree. or an integral multiple thereof in a sampling cycle (computation cycle), and GF indicates an injected fuel amount of the previous one stroke. Here, conventionally, the evaporation time constant .tau., which is what is called two-dimensionally mapped, is a parameter determined based on the operating conditions of the internal combustion engine, and this is stored in a memory of the control apparatus to make a suitable read-out structure. However, the above-described control apparatuses have the following problems which must be resolved.
Developmental Inefficiencies
Conventionally, the evaporation time constant .tau. is experimentally determined by operating the internal combustion engine based on various conditions, and produced as a two-dimensional map based thereon. Due to this, there are the disadvantages that many man-hours are required for this production and again, a great deal of labor is necessary for the correction of such maps. This means that development of a suitable fuel injection apparatus requires a great deal of labor and expense and is therefore inefficient.
Control problem at times of acceleration/deceleration
In addition, in the conventional control apparatuses, the above well-known equation is used as a C. F. Aquino equation to compute the fuel amount adhered to the intake system of the internal combustion engine per every crank angle of 720.degree. or per every multiple thereof, determining the fuel amount to be injected into the engine based on this obtained fuel amount. However, at times of transition where engine operating conditions change moment by moment even at intervals less than the crank rotation angle of 720.degree., especially during acceleration, deceleration and the like, the pressure of the intake system rapidly changes and the flow speed of air flowing into the intake system also changes rapidly, therefore the chronological change of the fuel amount actually flowing into the cylinder from the intake system differs from that at times of steady state operation. Due to this, in the conventional control apparatus which performs computation per every crank angle of 720.degree. based on the evaporation time constant .tau. at a constant operating time, there is the problem that the residual fuel amount at times of transition cannot be correctly understood and, ultimately, a suitable fuel injection amount at transitional times cannot be computed.
Control problem immediately after starting
Further, since a large portion of the fuel introduced into the cylinder adheres to the inner wall of the intake system (inner wall of the intake manifold and intake valve) and evaporates in the intake air flow, the evaporation time constant .tau. is greatly affected by the temperature of the inner wall of the intake system. The intake manifold has a direct heat transfer relationship with a cooling system of the internal combustion engine, therefore temperature changes thereof can be relatively easily known by measuring the temperature of the cooling water for example. Conventionally, control methods which measure the cooling water temperature and correct the fuel injection amount based thereon have been tried.
However, a certain proportion of the injected fuel also adheres to the intake valve. The intake valve temperature changes at a time constant remarkably smaller than the intake manifold temperature. Therefore, under conditions where the valve temperature differs from normal, such as for example immediately after start of operation of the internal combustion engine etc., there is the problem that the residual fuel amount in the intake system cannot be correctly recognized and a suitable fuel injection amount cannot be computed.